diff --git a/after/syntax/css/box-4.vim b/after/syntax/css/box-4.vim
new file mode 100644
index 0000000..f6771dd
--- /dev/null
+++ b/after/syntax/css/box-4.vim
@@ -0,0 +1,2 @@
+syn match cssBoxProp contained "\<margin-trim\>"
+syn match cssBoxAttr contained "\<in-flow\>"
diff --git a/test/test.css b/test/test.css
index 0ee5593..21f3b75 100644
--- a/test/test.css
+++ b/test/test.css
@@ -51,6 +51,11 @@
 	display: auto;
 }
 
+.box-4 {
+	margin-trim: auto;
+	display: in-flow;
+}
+
 .break-3 {
 	display: recto;
 	display: verso;